At present, in order to remove a fiber web from the outlet from a carder, it is known to use a small diameter take off cylinder which is adjacent to the last working cylinder of the carder, and which is rotated to have the same speed and the same direction as the last working cylinder. The last working cylinder may be a comb cylinder, for example, having the function of causing the fibers of the web to be parallel, or it may be a condenser cylinder having the function of tangling together the fibers of the web so as to increase the cohesion of the web in a direction extending transversely to the working direction of the carder.
Two main types of takeoff cylinder are known. In a first type, the outer surface of the cylinder is designed to enable the fiber web to attach to the entire periphery of the takeoff cylinder, while nevertheless ensuring that the web attaches thereto more weakly than it attaches to the last working cylinder. It may be constituted, for example, by a cylinder fitted with an isosceles covering, or by a cylinder having longitudinal fluting over its entire periphery.
The second known type of takeoff cylinder consists in a perforated cylinder having a suction sector that is stationary facing the last working cylinder. One such takeoff cylinder is described, for example, in French patent No. 1 500 746. When the fiber web reaches the suction sector, it is pressed against the periphery of the rotating takeoff cylinder. Beyond the suction sector, the fiber web ought theoretically to adhere no longer to the periphery of the takeoff cylinder. In practice, rotation of the takeoff cylinder gives rise to a surface peripheral suction flow downstream from the suction sector thus tending to hold the fiber web on the cylinder, which means that in the absence of additional web takeup means, the web winds up on the periphery of the suction cylinder.
Compared with a suction takeoff cylinder, the first above-mentioned type of takeoff cylinder has the main advantage of enabling the fiber web to be taken up more reliably from the periphery of the working cylinder. However, the price of such reliability is that the fiber web adheres to the periphery of the takeoff cylinder more strongly than it does to the periphery of the suction takeoff cylinder, beyond its suction sector.
With both known types of takeoff cylinder, it is necessary to use additional means for taking up the fiber web in order to direct the web towards the following processing operation: this may be constituted, for example, by an operation of consolidating the fiber web by passing it between two calenders.
One known way of taking up the web from the periphery of the takeoff cylinder is to cause it to pass between two rotating cylinders, or between the belt of a conveyor and a rotating cylinder located immediately above the conveyor. With such means, takeup of the web is necessarily accompanied by the web being stretched lengthwise. Unfortunately, a fiber web at the outlet from a carder has very low cohesion, including very low resistance to a transverse traction force. Consequently, when such a web is stretched lengthwise, the cohesion of the web is correspondingly reduced. As a result, beyond a maximum working speed of the carder, which at present is about 120 meters per minute (m/min), web stretching becomes excessive and the resulting web is of poor quality as to appearance, uniformity of weight, and isotrophy of its mechanical properties.
Proposals have also been made in patent U.S. Pat. No. 3,787,930 to take up the fiber web from the periphery of the takeoff cylinder ("doffer"), by holding it down by suction against the surface of a conveyor belt. The system described in that patent thus uses a takeoff cylinder, suction means, and a conveyor belt which is interposed between the suction means and the takeoff cylinder, and in which the belt is permeable to air.
The intended object of implementing that system is to redirect the fibers of the web in random manner during transfer by suction from the takeoff cylinder onto the conveyor belt, thereby obtaining a web with scrambled-together fibers at the outlet from the carder. To this end, suction is provided in the zone where the fiber web is taken up by the takeoff cylinder, in which zone the web is subjected to a reversal of direction. The suction means thus create a zone of turbulence in the web reversal zone, thereby enabling the fibers of said web to become scrambled. In that system, it is necessary to cause the conveyor belt to pass through the zone where the web is taken up by the takeoff cylinder at the outlet from the carder. Consequently, the portion of the conveyor belt that is used for receiving the web cannot be a rectilinear portion and it is necessarily a curved portion. Specifically, it is constituted more particularly by a portion of a cylinder. Further, in the system of patent U.S. Pat. No. 3,787,930, in order to consolidate the web, the peripheral speed of the takeoff cylinder is preferably chosen to be greater than the linear speed of the conveyor belt by at least 20%.